This application claims the priority of Japanese Patent Application No. 2000-309052 filed on Oct. 10, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens which is usable, when recording or reproducing information, for two optical recording media whose respective numerical apertures and wavelengths of light in use are different from each other, such that the different wavelengths of light in use are efficiently converged at respective desirable positions on their corresponding optical recording media; and an optical pickup apparatus using the same.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other as irradiation light.
Also, in the two optical recording media exemplified above, their numerical apertures have to be made different from each other due to their differences in characteristics. For example, numerical apertures of 0.6 and 0.45 have been set for the DVD and CD-R, respectively.
In order to reduce the thickness of a conventional optical pickup apparatus, as shown in FIG. 11, a mirror 51 for reflecting and deflecting the light from a light source is disposed, whereas an aperture stop 52, which is made of a liquid crystal shutter, a filter having a wavelength selectivity, or the like, is interposed between the mirror 51 and an object lens 53, so as to change the numerical aperture according to the difference in kinds of optical recording media 54.
However, the structure of the optical pickup apparatus becomes complicated when the above-mentioned aperture stop is used, which opposes the demand for lowering the cost. In particular, units for recording/reproducing DVD and CD-R used in notebook type personal computers have been required to reduce the thickness of their optical pickup apparatus by decrements of {fraction (1/10)} millimeters. The thickness reduction in optical pickup apparatus using an aperture stop has already reached its limit.
In view of the foregoing circumstances, it is an object of the present invention to provide a compact, low-cost objective lens for optical recording media, which can attain respective numerical apertures corresponding to two optical recording media without complicating the structure of the optical pickup apparatus; and an optical pickup apparatus using the same.
For achieving the above-mentioned object, the present invention provides an objective lens for optical recording media, which, when recording or reproducing information for a first optical recording medium corresponding to a first numerical aperture and a first wavelength and a second optical recording medium corresponding to a second numerical aperture and a second wavelength, converges the wavelengths of light in use to respective desirable positions;
wherein one surface of the objective lens is provided with a zonal part which acts so as to apparently cancel the quantity of light having one wavelength xcex1 in an outer peripheral portion of a luminous flux thereof due to an interference effect but keep the quantity of light having the other wavelength 2 in the outer peripheral portion thereof;
wherein the zonal part has such a step that light supposed to pass a position corresponding to the zonal part if the zonal part does not exist and light passing a position at which the zonal part is formed if the zonal part exists generate therebetween a phase difference satisfying the following conditional expressions (1) and (2):
xcex94xcex1=(2m+1)xcex1/2+xcex41xe2x80x83xe2x80x83(1) 
xcex94xcex2=nxcex2xcex42 xe2x80x83xe2x80x83(2) 
where
xcex94xcex1 is the phase difference at the wavelength xcex1 between the light supposed to pass the position corresponding to the zonal part if the zonal part does not exist and the light passing the position at which the zonal part is formed if the zonal part exists;
xcex94xcex2 is the phase difference at the wavelength xcex2 between the light supposed to pass the position corresponding to the zonal part if the zonal part does not exist and the light passing the position at which the zonal part is formed if the zonal part exists;
m and n are integers;
|xcex41| less than 0.2 xcex1; and
|xcex42| less than 0.2 xcex2; and
wherein the numerical aperture at a boundary position of the step has a value between the first and second numerical apertures.
The zonal part may be formed such that the objective lens surface has a center part recessed or protruded with respect to the outer peripheral face thereof.
As a consequence of such a configuration, concerning light having the wavelength xcex1, the phase difference between the light having passed through the part of objective lens other than the zonal part and the light having passed through the zonal part becomes an odd multiple of xc2xd wavelength, so that the light having passed through the part other than the zonal part and the light having passed through the zonal part interfere with each other, whereby their quantity of light apparently disappears. (While the fundamental explanation is provided in Teruji Ose, xe2x80x9cFeature: Resolution Limit Overcoming Technique Expected for Practicexe2x80x94Ultra Resolution Optical System,xe2x80x9d O plus E (September 1992), pp. 66-72, it will be explained here in terms of interference of light.)
Concerning light having the wavelength xcex2, the phase difference between the light having passed through the part other than the zonal part and the light having passed through the zonal part becomes an integral multiple of the wavelength, thereby yielding no apparent phase difference, so that the quantity of light is maintained as it is in the outer peripheral portion of the luminous flux.
For example, in the case where the wavelength xcex1 used for CD systems such as CD-R is 780 nm, whereas the wavelength xcex2 used for DVD is 650 nm, the quantity of light apparently disappears in the outer peripheral portion of the luminous flux in the wavelength light used for CD systems but is kept as it is in the wavelength light used for DVD.
In this case, it will be adequate if the step in the zonal part is set, for example, such that a phase difference which is an integral multiple of the wavelength occurs at 650 nm whereas a phase difference which is an odd multiple of the xc2xd wavelength occurs at 780 nm.
If an optical path difference exists between the light passing through the part other than the zonal part and the light passing through the zonal part, an interference effect will occur therebetween so as to weaken them. When light having a Gaussian amplitude distribution enters the lens, however, it is preferable for the phase differences to be changed rather than being generated at an odd multiple of the xc2xd wavelength concerning the light having the wavelength B, and at an integral multiple of the wavelength concerning the wavelength xcex2, in order to cause an interference effect for the light having the wavelength xcex1 and attain a desirable numerical aperture. The permissible amount of change is up to about 20%.
For example, when the numerical aperture with respect to the wavelength xcex1 used for CD systems such as CD-R is 0.45 whereas the numerical aperture with respect to the wavelength xcex2 used for DVD is 0.6, it is considered preferable if the boundary position of the zonal part is located at a position where an area whose numerical aperture NA ranges from 0.45 to 0.6 is divided into two equal parts.
Letting NA be the numerical aperture, and f be the focal length, the area of the lens surface at the numerical aperture NA becomes xcfx80(NAxc2x7f)2. Therefore, letting NAx be the numerical aperture at the boundary position in the zonal part for switching the numerical aperture between 0.6 and 0.45,                     π        ⁡                  (                      0.6            xc3x97            f                    )                    2        -                  π        ⁡                  (                      NAx            xc3x97            f                    )                    2        =            π      ⁢                        (                      NAx            xc3x97            f                    )                2              -                            π          ⁡                      (                          0.45              xc3x97              f                        )                          2            .      
Hence, the numerical aperture NAx at the boundary position of the zonal part is:                     NAx        =                  xe2x80x83                ⁢                              {                                          [                                                                            (                      0.6                      )                                        2                                    +                                                            (                      0.45                      )                                        2                                                  ]                            xc3x97                              (                                  1                  /                  2                                )                                      }                                1            /            2                                                  ≈                  xe2x80x83                ⁢                  0.53          .                    
Namely, since the boundary of the zonal part is provided at a position where the numerical aperture becomes about 0.53, the phase difference between the light having passed through the zonal part and the light having passed through the part other than the zonal part becomes an odd multiple of the xc2xd wavelength concerning the light having a wavelength of 780 nm, so that they interfere with each other, thereby apparently canceling the quantity of light in the outer peripheral portion of the luminous flux, by which the numerical aperture becomes about 0.45. For the light having a wavelength of 650 nm, there is no apparent phase difference between the light having passed through the zonal part and the light having passed through the part other than the zonal part, whereby the numerical aperture of the object lens is exhibited as it is, so that the numerical aperture becomes 0.6.
Here, in the case where the numerical aperture of the lens is set to 0.45 for CD systems such as CD-R and 0.6 for DVD as mentioned above, for example, the zonal part may be a recessed or protruded zone having an outer boundary position, other than the boundary position with respect to the center part, located on the outer side of a position where the numerical aperture becomes 0.6. i.e., the greater numerical aperture value. Alternatively, the zone may be such that no outer boundary position exists up to the edge face of the lens.
Also, the present invention provides an optical pickup apparatus comprising the objective lens for optical recording media.
Though Japanese Unexamined Patent Publication No. 10-26726 discloses an objective lens whose surface on the light source side is provided with a zonal part, this is an objective lens for an optical pickup to be used with a single-wavelength light source. Also, the objective lens disclosed in this publication is aimed at reducing the large wavefront aberration generated when an optical system for DVD is used for CD systems. Namely, the objective lens disclosed in this publication is not applicable to double-wavelength light sources.